Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disorder in which selective loss of motor neurons leads to fatal paralysis. Military personnel are nearly twice as likely to develop ALS, regardless of the branch of service or the time period served, thus its impact on veteran health care is enormous. However, little is known about the pathogenesis of ALS, and there is no cure or effective treatment. Discovery of mutations in SOD1 has been the driving force in ALS research for the last two decades. More recently, two RNA binding proteins (RBP), TDP-43 and FUS, have been linked to sporadic and familial ALS, and underscore the role of abnormal RNA processing in the pathogenesis of this disease. How TDP-43 and FUS are regulated is unknown. We have found that Human antigen R (HuR) is required for proper TDP-43 and FUS expression. HuR is a major RBP with striking structural and functional similarities to TDP-43 and FUS. We have reported that cellular toxicity induced by mutant SOD1, including apoptosis and mitochondrial dysfunction, could be reversed by up-regulating HuR in a gene dose-dependent manner, suggesting a protective role of HuR in neurodegeneration. Our preliminary data suggest that HuR translocates to the cytoplasm in spinal cords of ALS patients, it specifically binds to TDP-43 3'UTR and promotes TDP-43 and FUS expression under normal and stressed conditions. HuR also regulates ubiquitination of cellular proteins, a key element in the proper functioning of the proteasome. We hypothesize that HuR positively regulates TDP-43 and FUS expression through posttranscriptional mechanisms. Disruption of this regulation leads to compromised cellular stress response and motor neuron toxicity. We will test this hypothesis by performing experiments with the following specific aims: 1. Define HuR as an upstream regulator of TDP-43 and FUS, and assess its downstream effect on TDP-43 splicing function, cellular stress response and motor neuron survival. 2. Determine the role of cis-regulatory elements in the 3'UTR of TDP-43 in its cross- and self- posttranscriptional regulation, and analyze protein-protein and protein-RNA interactions between HuR and TDP-43. 3. Determine the role of HuR in the translational regulation and protein degradation of TDP-43 and FUS.